This is another one of Heinlein's 'juvenile' books. While is has some good sci-fi concepts, it is overall one of the lower quality Heinlein books of the early period. It would not make much sense unless you already knew a lot about science fiction writing in general and Heinlein specifically.
There are a few redeeming features. The passages written from the point of view of the alien 'Lummox' are well-written. The portrayal of humans dealing with truly alien cultures is good and thought-provoking. But the book overall has a lot of problems.
The teenage boy who is supposed to be the main character is a non-entity. He is fairly clueless, and incapable of logic, thought, or planning. He doesn't really do anything except follow the orders of other people, and occasionally give into an irrational impulse that leads him to defy that authority. His girlfriend, by contrast, is extremely intelligent and competent, and a far more lively and heroic character. I have no idea what she sees in him; their relationship is just assumed, like a fact of nature. He certainly doesn't do anything to earn her love or respect.
This kind of thing is disturbingly common in Heinlein books. He makes female characters who are smart, intelligent, capable, etc., but who have absolutely no ability to apply their intelligence to their personal life, and fall in love with random people for reasons that are never explained. He may think that he is making progressive female characters, but all he is really doing is creating a fantasy girlfriend for the hero.
Or maybe this is just another symptom of the fact that Heinlein always had a strange and insidious view of love. It is always portrayed as some kind of irresistible destiny that operates independently of what people do. In his first published book, one of the characters said something like "If you ever really had a chance with her, she isn't going to let a little thing like you trying to shoot her get in the way." That's just outrageous. It basically implies that women are idiots who will forgive any kind of bad behavior if they love you. Heinlein often seems to imply that the love of a women is something random, not something you earn.
Setting that aside, the actual main characters of the book are not the two teenagers. A pair of government agents who are dealing with an alien negotiator get more words written about them, and are also far more important to the plot. This part of the book actually makes a decent story. It is almost as if Heinlein had a good idea for a story, but had a contract to write a juvenile book, so he had to add in the two teenagers as an afterthought to satisfy the publisher.
The book does have interesting little hints about a different social order. I suspect that the book was censored in some way, or that Heinlein wrote it carefully to try to get messages across while avoiding censorship. It is strongly implied that gender roles have reversed. For example, if a boy and a girl go out without a chaperone, then it is the boy's reputation that is put at risk. But even this breaks down at the end. As events unfold and it is revealed that the two kids will get married, the girl's primary ambition is to secure a job for her husband that has the status and pay artifiially inflated. She, despite being more qualified in every possible way, does not seem to get or want a job.